Surfcraft Fin Arrangement

ABSTRACT

A fin arrangement for a surf craft includes a base coupling portion to couple the fin arrangement to the surf craft; a central fin having a central fin base portion, the central fin coupled to the base coupling portion to extend substantially perpendicularly away from an underside of the surf craft; a first side fin coupled to the central fin base portion and extending away from the central fin and the surf craft at a first acute angle to the central fin; and a second side fin coupled to the central fin base portion and extending away from the central fin and the surf craft at a second acute angle to the central fin, the second acute angle being substantially the same as the first acute angle, and a projecting lobe extending rearwardly from the base of the central fin.

TECHNICAL FIELD

Described embodiments generally relate to fin arrangements for a surf craft, such as a surfboard, and to surfcraft including such fin arrangements. More specifically, embodiments relate to fin arrangements comprising three fins.

BACKGROUND

The following background discussion is intended to facilitate an understanding of described embodiments only. This description is not intended to be an acknowledgement or admission that any of the information and material referred herein was published, known or part of the common general knowledge of the inventor or any person skilled in the art of watercraft fin shape, design or technology in any jurisdiction as at the priority date of the invention.

A surfboard fin is a hydrofoil mounted on the underside of the tail of a surfboard to provide directional stability and control whilst moving in a linear forward motion through the water utilizing foot-steering by the rider of the surfboard.

Surfboard fins can provide lateral lift opposed to the water and stabilize the surfboard's trajectory, allowing the surfer to control direction by varying their side-to-side weight distribution. The introduction of fins in the 1930s revolutionized surfing and board design. Surfboard fins have been arrayed in various numbers and configurations, and many different shapes, sizes, and materials are and have been made and used. In 1980, Simon Anderson (Australia) introduced the “three fin” or “Thruster” design, which has been standard ever since.

In surfing, there are two major types of typically stationary surfboard fins or hydrofoils, and a host of illustrative issues. The “rail fins” stabilize the motion of the surfboard. It also contributes to the desired effect of converting the kinetic energy or push of the sloped wave face combined with the rider's mass or potential energy into redirected energy or lift. The surfer deflects the surfboard and fins off the water of the wave face (and/or vice-versa) to make forward progress across the wave face, or “down the line”, that is, parallel to the wave crest and beach. Riding parallel to the crest, which is perpendicular to the pull of gravity down the wave's slope, in this way is known as “trimming” (aka “drive”) from the board and its fin(s) is what enables such manoeuvres in surfing.

A fin is an upright, streamlined and often raked or swept keel, which may be provided in the form of a centrally mounted stabilizer foil mounted perpendicularly to the riding surface at the rear or “tail” of the surfboard.

Smaller surfboard fins mounted near the edge (or “rail”) of the surfboard are known as “rail fins” and are seen in multi-fin arrangements, often in combination with a similarly sized central fin further back on the board. Rail fins enable high-performance surfing, and are most often “single foiled”, with one flat side and one “foiled” side as seen on an aircraft foil, for greater lift. In recent times, fin manufacturers including fin shapers have also produced fins with a concave surface as opposed to the flat high pressure surface.

A fin configuration with fins near the edge of the board stabilizes and contributes lift during turning manoeuvres, which contributes to the board's ability to “hold” during turning manoeuvres.

Rail fins are often seen in addition to a central fin, but can be used without a central fin as well. Some multi-fin configurations use two rail fins (a “twin-fin”), two rail fins plus a similar-sized central fin mounted further back (e.g. a “Thruster”), or four fins (a “quad”). Rail fins are more or less engaged by the rider's heel and toes as they lean in the desired direction of their turn. As the rider does so, an “inside” rail fin sinks deeper and its angle of attack is increased, as is its lift-induced drag. Rail fins also add lift (known as “drive”) in trim and with greater holding ability, enable steeper wave faces to be ridden and higher speed “down the line.”

Rail fins are typically “toed-in,” that is, the leading edge of the fins are oriented toward the centreline of the surfboard, which decreases the angle of attack in trim, which makes it easier to initiate turns. “Toeing in” rail fins also adds drag on the “outside” fin, as its angle of attack is negative during trim or in a turn. These combined factors of toed-in rail fins cause several issues: drag on a toed-in outside rail fin can slow the board down in trim, but it can also give a braking effect during turns that is useful. The inside rail fin (and the board itself) can be “pumped,” attacked and re-attacked, by swerving up and down the face, causing acceleration down the line, or similarly pumped to achieve a desired trajectory through a multi-stage turn. At higher speeds, the drag off toed-in rail fins can cause surfboards to oscillate and become unstable—a phenomenon known as “speed wobbles”.

Most surfboards intended for larger waves are longer (to increase hull speed for paddling, wave-catching, and surfing), and as most surfboard shapers orient the rail fins toward the nose of the board, a longer board inherently results in reduced toe-in of rail fins, therefore less negative angle of attack, less oscillation, greater stability, and higher speeds. Rail fins also typically have some degree of “cant,” that is, are tilted out toward the rail they are adjacent to. This is a significant additional factor in lift at various attitudes, drag, and performance, as are the variables of other foils—including flexibility, thickness, and plan form.

Rail fins evolved into being and came into popularity as riders (Simon Anderson, most famously) sought a solution to two major performance issues of a central “single” fin—both related to engagement of the foil: for one, a centrally-mounted fin is tilted up out of the water as the board is leaned over, and thus it loses more and more of its lift as the lean angle increases. If the lean angle is acute enough, the fin's tip can be the only area left in the water; the tip may then rapidly stall and, having lost its lift, become disengaged from the water, leaving the board's bottom as the only control surface still operating. Before rail fins became extremely popular, this tendency of “single fins” led to riders “nursing” turns—this tendency was a significant limiting factor on performance. The enhanced hold offered by rail fins during turning led to more types of manoeuvres being possible. The other major issue leading to rail fins' use is the fact that a rider can use the lift near the rail to increase speed and performance on smaller waves due to the above effects and abilities of these foils.

The “Thruster” is a tri-fin arrangement in which the fins are the same size, with two semi-parallel (slightly toed-in, usually, and slightly canted outward, usually) fins mounted near the rails 10 inches to 12 inches forward of the tail and a middle fin at 3 inches to 5 inches. The “Thruster” fin design is constructed to reflect an inverted triangular configuration situated at the rear or tail end underside of the surfboard.

Additionally, the asymmetrical side fins are constructed with a single convex low pressure surface and a flat high pressure surface. In recent times, fin manufacturers including fin shapers have also produced fins with a concaved surface as opposed to the flat high pressure surface. Most surfboard fins consist of a moderately swept plan form whose foiled base generally has a thickness to chord (t/c) ratio of between 6% to 10%. Furthermore, the Thruster fins are set with a cant angle of between 2% to 10% to ensure the lift force acts mainly in the horizontal direction during turns during turns into the face of the wave, as well as a toe angle of between 0.5% to 10% to achieve zero horizontal lift while in a linear forward motion in the water.

Furthermore, the inverted triangular configuration of the trailing and leading fins on the “Thruster” provides a large “pivot area” or “turning arc area”, which may have the following example dimensions:

Distance: Leading Edge to Leading Edge on Leading Fins=29 cms (a) Distance: Leading Edge on Leading Fin to Leading Edge on Trailing Fin=24 cms (b) Distance: Trailing Edge on Leading Fin to Leading Edge on Trailing Fin=18 cms (c) Distance: Trailing Edge on Leading Fin to Trailing Edge on Trailing Fin=25 cms (d)

Thus the total Area between each fin is triangulated as follows:

Leading Edge on Leading Fin to Leading Edge on Trailing Fin=348 cms Trailing Edge on Leading Fin to Leading Edge on Trailing Fin=261 cms Trailing Edge on Leading Fin to Trailing Edge on Trailing Fin=362.5 cms

The large pivot area of the “Thruster” provides for a wide turning area or arc as the rider manoeuvres through a turn and may allow for the rear foot of the surfer to be positioned near the trailing fin at the tail end of the board.

It is desired to address or ameliorate one or more shortcomings or disadvantages of prior surfcraft fins or arrangements, or to at least provide a useful alternative thereto.

Throughout this specification the word “comprise”, or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.

Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present disclosure as it existed before the priority date of each claim of this application.

SUMMARY

Some embodiments relate to a fin arrangement for a surf craft, comprising:

a base coupling portion to couple the fin arrangement to the surf craft;

a central fin having a central fin base, the central fin coupled to the base portion to extend substantially perpendicularly away from an underside of the surf craft;

a first side fin coupled to the central fin base and extending away from the central fin and the surf craft at a first acute angle to the central fin; and

a second side fin coupled to the central fin base and extending away from the central fin and the surf craft at a second acute angle to the central fin, the second acute angle being substantially the same as the first acute angle.

The first and second acute angles may be between about 30° and about 60°. Optionally, the first and second acute angles may be between about 35° and about 55°. Optionally, the first and second acute angles may be between about 40° and about 50°. Optionally, the first and second acute angles may be between about 43° and about 47° or optionally about 45°.

The central fin may have a lateral width of about 9 mm to about 15 mm at a base of the central fin. The lateral width of the central fin may be non-uniform in a fore-aft direction and in a base-to-tip direction. A mean thickness to chord (T/C) ratio of the central fin may be about 0.10 to about 0.13. The T/C ratio may vary from about 0.090 near a base of the side fin to about 0.143 toward a tip of the side fin.

Embodiments also relate to a surf craft fin arrangement, comprising:

a central fin to extend substantially perpendicularly away from an underside of the surf craft;

a first side fin coupled to a central fin base portion or to the surf craft underside at a position closely adjacent to the central fin base portion, the first side fin extending away from the central fin and the surf craft at a first acute angle to the central fin; and

a second side fin coupled to the central fin base portion or to the surf craft underside at a position closely adjacent to the central fin base portion, the second side fin extending away from the central fin and the surf craft at a second acute angle to the central fin, the second acute angle being substantially the same as the first acute angle.

Some embodiments relate to a fin arrangement for a surf craft, comprising:

a base portion to couple the fin arrangement to the surf craft; a central fin coupled to the base portion to extend substantially perpendicularly away from an underside of the surf craft; a first side fin coupled to the central fin and extending away from the central fin and the surf craft at a first acute angle to the central fin; and a second side fin coupled to the central fin and extending away from the central fin and the surf craft at a second acute angle to the central fin, the second acute angle being substantially the same as the first acute angle, wherein at least the central fin comprises a projecting lobe extending rearwardly from a base portion of the central fin.

The first side fin and second side fin may each comprise a projecting lobe extending rearwardly from base portions thereof.

The side fins are positioned to extend away from opposite sides of the central fin.

Some embodiments relate to a surf craft comprising one of the fin arrangements described above.

Some embodiments relate to a fin for a water craft, the fin comprising a base, a convex leading edge, a rounded tip, a trailing edge which is at least partially concave, and a projecting lobe extending aftwardly from a base portion of the fin, wherein the trailing edge of the projecting lobe is convex, and wherein a chord of the base is the longest chord of the fin.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a front perspective view of a fin arrangement according to some embodiments;

FIG. 2 is a top view of the fin arrangement of FIG. 1;

FIG. 3A is a side view of the fin arrangement of FIG. 1;

FIG. 3B is a front view of the fin arrangement of FIG. 1;

FIG. 4 is a perspective view of a fin arrangement coupled to the underside of a surfcraft according to some embodiments;

FIG. 5A is a front view of a fin arrangement according to some embodiments;

FIG. 5B is a bottom view of the fin arrangement of FIG. 5A;

FIG. 6 is a side view of a side fin according to some embodiments;

FIGS. 7A to 7I are cross sections of the side fin of FIG. 6;

FIG. 8 is an upper side perspective view of the fin arrangement of FIG. 1;

FIG. 9 is an image of a Computational Fluid Dynamics (CFD) model of a “thruster” fin arrangement showing the lift force acting on the fins and surfcraft;

FIG. 10 is a front perspective view of a fin arrangement according to some embodiments;

FIG. 11 is a front view of the fin arrangement of FIG. 10;

FIGS. 12A to 12D are foil sections of a side fin according to some embodiments;

FIG. 13 is an image of a CFD model of the fin arrangement of FIG. 10 showing the fluid pressure acting on the surfaces of the fin arrangement and surfcraft;

FIG. 14 is an image of a CFD model of the fin arrangement of FIG. 10 showing the lift force acting on the surfaces of the fin arrangement and surfcraft;

FIG. 15 is an image of a CFD model of the fin arrangement of FIG. 10 showing the drag force acting on the surfaces of the fin arrangement and surfcraft;

FIG. 16A is a rear perspective view of a fin arrangement according to some embodiments;

FIG. 16B is a front perspective view of the fin arrangement of FIG. 16A;

FIG. 17A is a front view of the fin arrangement of FIG. 16A;

FIG. 17B is a top view of the fin arrangement of FIG. 16A;

FIG. 17C is a side view of the fin arrangement of FIG. 16A;

FIG. 18 is a side view of a side fin according to some embodiments;

FIGS. 19A to 19H are parallel foil sections of the side fin of FIG. 18;

FIG. 20 is a side view of a side fin according to some embodiments;

FIGS. 21A to 21G are angled foil sections of the side fin of FIG. 20;

FIG. 22 is a side view of a centre fin according to some embodiments;

FIGS. 23A to 23G are parallel foil sections of the centre fin of FIG. 22;

FIG. 24 is a two dimensional outline of a side fin according to some embodiments;

FIG. 25 is a two dimensional outline of a centre fin according to some embodiments;

FIG. 26A is a front perspective view of a fin arrangement according to some embodiments;

FIG. 26B is a rear perspective view of the fin arrangement of FIG. 26A;

FIG. 27A is a two dimensional outline of a centre fin according to some embodiments;

FIG. 27B is a top view of the fin arrangement of FIG. 26A;

FIG. 27C is a front view of the fin arrangement of FIG. 26A;

FIG. 27D is a side view of the fin arrangement of FIG. 26A;

FIG. 28 is a side view of a centre fin according to some embodiments; and

FIGS. 29A to 29H are parallel foil sections of the side fin of FIG. 28.

DETAILED DESCRIPTION

Embodiments of a fin arrangement for a surfcraft are now described in further detail below, with reference to FIGS. 1 to 8. While the term fin is used herein to generally indicate a hydrofoil structure and function, other terms, such as skeg and blade, may also be used to indicate similar structures and functions. The embodiments described herein are intended to be generally positionally fixed or static and non-motorised.

Described embodiments relate generally to a three-bladed trident-shaped fin arrangement or assembly, with the fins joined and intersecting at a base. The fin arrangement may also be described as a tri-fin arrangement. In such embodiments, the two symmetrical leading fins on the rails of the “Thruster” and its centre trailing fin are reconfigured and brought together into the one unit, as opposed to three fins positioned separately and spaced apart in an inverted triangular configuration near the tail of the surfboard as seen on the “Thruster”. In some embodiments, a base coupling portion of the three-bladed fin arrangement may be slid or slotted in and engaged with a fin box that is fixed into a surfcraft.

For convenience only, and without limitation, the following terms are used in this description. Described embodiments generally relate to a fin arrangement comprising a centre fin and two (or possibly more) side fins. Each fin comprises a leading edge (the foremost edge of a fin), a trailing edge (the aftmost edge of a fin) and a base. For the side fins, the base is where the fin couples to the centre fin or, in some embodiments, to the surfcraft. For the centre fin, the base is where the fin couples to a coupling portion that is attached or attachable to the surfcraft.

The two dimensional profile of a fin described by the leading edge, trailing edge, and base may be referred to as a fin outline. Two surfaces may be described for each fin, bounded by the leading edge, trailing edge, and base. The surface of a side fin nearest the centre fin may be referred to as an inner surface and the surface of a side fin farthest from the centre fin may be referred to as an outer surface.

For the purposes of describing fin geometry, the following characteristics may be referred to. The surfaces of each fin may be illustrated with a series of foil sections showing the two dimensional profile of a fin seen in cross-section parallel to the base of the fin and normal to the surfaces of the fin. The longest straight line in a foil section from the leading edge to the trailing edge may be referred to as a chord. A plane coinciding with and parallel to the chords of the centre fin may be referred to as the central plane. An axis collinear with the chord of the base may be referred to as a base axis. The point on a fin farthest from the base axis where the leading edge and trailing edge meet may be referred to as a fin tip. The distance of the fin tip from the base axis may be referred to as the base to tip distance. The shortest straight line which can be drawn from the base axis to the fin tip may be referred to as the base to tip line. The angle between the base to tip line of a side fin and the central plane may be referred to as the cant or cant angle. The aftmost point of a fin in the tip portion may be referred to as the trailing tip. The angle between the base axis aft of the trailing edge and a line connecting the trailing edge of the base to the trailing tip may be referred to as the rake or rake angle. The angle of a side fin base chord relative to the centre fin base axis may be referred to as the angle of attack.

The portion of a fin less than approximately one third of the base to tip distance from the base axis may be referred to as the base portion. The portion of a fin more than approximately two thirds of the base to tip distance from the base axis may be referred to as the tip portion. The portion in between the base portion and the tip portion may be referred to as the intermediate portion.

The fin arrangement comprises a central fin and two side fins. The fin arrangement may be symmetrical about the central plane. The side fins may extend away from the central fin with a cant angle of between about 30° and 65°, optionally about 40° and 60°, optionally about 45° and 55°, optionally about 52°.

The bases of the side fins may be fixed near the base of the central fin. In some embodiments the side fins may be joined to the centre fin, or alternatively, the side fins may be joined to the surface of a surfcraft. The centre fin and side fins may be integrated into a single tri-fin, or may be independently coupled to a surfcraft. The distance of the base trailing edge of each side fin from the base axis of the centre fin may be less than 30% of the base to tip distance of the centre fin, optionally less than 15% of the base to tip distance of the centre fin, optionally less than 10% of the base to tip distance of the centre fin, optionally less than 5% of the base to tip distance of the centre fin.

The fins may comprise hydrodynamically efficient curved surfaces in order to enhance control and performance characteristics and mitigate drag. The fins may generally taper from base to tip. In some embodiments, the chord length may decrease from base to tip. In other embodiments, for two or three sequential chords, the length of a succeeding chord may the same or greater than the length of the preceding chord. The foil section thickness may generally decrease from base to tip. The foil section shape may vary from base to tip. The foil sections may have a number of different suitable profile shapes, while still achieving the advantages of the tri-fin arrangement. For example, the foil sections may be generally thicker in the fore portion compared with the aft portion of each foil section, with a rounded leading edge portion and a tapered trailing edge portion. The centre fin foil sections may be symmetrical about their chords or central plane. The side fin foil sections may be symmetrical about their chords or may be asymmetrical. The inner surfaces of the side fins may be substantially flat, or alternatively may be curved, and may be at least partially concave.

The fin outline of the centre fin and side fins may comprise a number of different suitable fin shapes, while still achieving the advantages of the tri-fin arrangement. For example, the leading edge may be a convex curve, the tip portion may be rounded, and the trailing edge may be at least partially concave. The trailing edge may have a convex curvature near the tip portion and a concave curvature near the intermediate and base portions. The fin may be swept aftward and the centreline of the fin outline may be curved.

The centre fin and/or the side fins may comprise a projecting lobe extending aftwardly from the base portion. The projecting lobe may also be described as a spur, beak, horn, nose, protrusion, projection, protuberance or extension. The trailing edge of the projecting lobe may have a convex curve starting at a base trailing edge, the base chord being the longest chord of the base portion. The convex curve of the trailing edge of the projecting lobe may intersect the concave curve of the trailing edge of the intermediate portion to provide a somewhat sharp transition point therebetween. The intersection may be positioned at a location approximately one third of the base to tip distance from the base axis. The trailing edge curve of the projecting lobe may continue past the intersection point towards the leading edge until the thickness of the projecting lobe matches the thickness of the intermediate portion of the fin which may be referred to as a matching point. There may be a transition step in the region between the intersection point and matching point where the thickness of the projecting lobe does not match the adjacent intermediate portion. The transition step may be smoothed with rounded edges and reduce gradually in depth from the intersection point to the matching point.

In some embodiments either the centre fin or the side fins comprise a projecting lobe as described above. In other embodiments, each of the side and centre fins comprise respective projecting lobes.

In some embodiments, a single fin may be provided with a fin outline having a leading edge with a convex curvature, a rounded tip, a trailing edge with convex curvature near the tip portion and concave curvature near the intermediate portion, and a projecting lobe extending from the base portion with a convex trailing edge. The longest chord of the base portion may be the base chord. The foil sections may be symmetrical about the chords with hydrodynamically efficient profiles.

Referring to FIGS. 1 to 4 and 8, a fin arrangement is shown according to some embodiments. The fin arrangement 100 is composed of two symmetrical side fins 130, 140, being mirror images in profile and configuration and extending at a maximum depth of 144.01 mm. The side fins 130, 140 have a base length of about 128 mm from the leading edge 132, 142 to the trailing edge 134, 144. The centre fin 120 has leading edge 122 and a trailing edge 124 converging at a tip 126, and a maximum depth of 160 mm. The central fin is symmetrical around its central plane and chords, with generally hydrodynamically efficient side surfaces 127, 128. The inner surface 138, 148 of each of the side fins 130, 140 is not flat and is not symmetrical with the outer surfaces 137, 147, as illustrated in FIGS. 6 and 7A to 7I.

Each of the fins 120, 130, 140 is fixedly coupled (e.g. integrally formed by moulding, for example) to a base region 105 that has a coupling portion 110 extending therefrom to allow the fin arrangement 100 to be coupled to a surfcraft, for example as illustrated in FIG. 5. Each side fin 130, 140 has a “rake” or “sweep” of 65.4 degrees, as shown in FIG. 8. The foil sections shown in FIGS. 7A to 7I are taken at various angles relative to the base, as shown in FIG. 6, and do not represent parallel chords. However, the length of each such foil section is referred to as a chord for convenience. T/C (thickness to chord) ratios along 9 cross sections (shown by FIGS. 7A to 7I, respectively) or “foils” on the side fins 130, 140 as follows:

Foil Section Ratio % Section 1-1 0.090 Section 2-2 0.096 Section 3-3 0.105 Section 4-4 0.113 Section 5-5 0.116 Section 6-6 0.121 Section 7-7 0.123 Section 8-8 0.129 Section 9-9 0.143 Mean t/c ratio: 0.115

Each of three fins 120, 130, 140 on the multi-fin arrangement 100 intersect at a base 105, with a cant angle X (see FIG. 4) from the base to tip line 135, 145 (which might alternatively be referred to as a chord line) on each of the side fins 130, 140 to the central plane 125 (which might alternatively be referred to as a chord line) of the centre fin 120. The cant angle may be about 35 degrees to about 65 degrees, optionally about 40 degrees to about 60 degrees, optionally about 45 degrees to about 55 degrees and optionally about 50 degrees. This represents an increase in cant angle of between 40% to 48% compared to the current “Thruster” configuration. Unlike the “Thruster”, this cant angle allows for vertical lift in addition to the lateral lift provided by the “rail” fins of the Thruster configuration. In other words, the “Thruster” only allows for unilateral lift in the horizontal angle to the face of the wave whereas the described multi-fin arrangement 100 allows for lift in both the horizontal and vertical directions. This provides for increased manoeuvrability and according to testing in variable and diverse small and large wave conditions, a feeling of increased “freedom” for the surfer.

This is achieved as a direct result of the unevenness of the foil in the side fin as shown by Sections 1-1 to 9-9 in FIGS. 7A to 7I.

An upper surface area 137, 147 of each of the side fins 130, 140 may be about 24450-24460 mm2.

As a surfboard moves forward into a turn, the centre fin commences at approximately vertical to the face of the wave and arcs counter clockwise at 90 degrees where it become more horizontal. At this point, a normal centre fin is unable to hold into the wave and stalls as a result of the acute angle it assumes through the process of the turn. This problem is addressed by described embodiments.

As a result of the side fins 130, 140 positioned at the intersection of the centre fin 120 on the base 105 that allows for vertical lift to be achieved in all wave conditions that as the centre fin and one of the side fins stalls horizontally to the wave face, the remaining side fin attaches or “holds” more effectively in and to the water thus preventing the tail end of the surfboard from slipping or sliding along the wave. However, the performance of the board is still maintained at this point even after the centre fin has stalled as one of the side fins 130, 140 provides immediate “hold” into the wave, thus preventing “popping” when the tail end becomes aerial and thus the surfboard leaves the water.

Additionally, the resulting vertical lift of described embodiments allows for immediate and dramatic change in direction with a significantly reduced pivot area or turning arc thus further increasing the sense of freedom experienced by the surfer.

Referring also to FIG. 4, an example surfcraft 400 is illustrated. The surfcraft comprises a surfboard 405, with the fin arrangement 100 disposed in a fixed position on the underside 410 of the surfboard 405. The base area of the base 105 of fin arrangement 100 represents the total area of pivot or turning arc and is defined by its base length, depth and height of 130 mm×12.50 mm×8.90 mm respectively. The base length of the side fins 130, 140 may be slightly shorter than the centre fin 120, for example by a few millimetres (i.e 128 mm, for example). The surfer's back steering foot will commonly be on and over the part of the board under which is positioned the fin arrangement 100 (i.e on underside 410), which allows the surfboard to respond more quickly, thus performing sharper turns or tighter arcs, which further enhances the overall manoeuvrability and performance of the board.

In some embodiments, the fin arrangement 100 may be detachable and movable as shown in the drawings or in alternative embodiments it may be integrally formed with the surfboard 405, in which case the side fins 130, 140 may be coupled to the central fin 120 or possibly coupled instead to the board underside 410 at a position closely adjacent to either side of the base of the central fin 120, so as to effectively provide a similar hydrodynamic profile to the fin arrangement 100.

However, fin arrangement 100 eliminates two out of the three base lengths, leaving one remaining fin in its box. This fin arrangement 100 may be positioned approximately 200 mm from the tail 415 of the board 410 to the trailing edge and 330 mm to its leading edge 122.

The fin arrangement 100 may be fixed within a slotted box 420 in a surf craft 410 as shown in FIG. 4. The longitudinal position of the fin arrangement 100 within the slotted box 420 may be adjustable by around 70 mm, allowing a surfer to adjust the turning characteristics of the surfboard.

Now referring to FIGS. 5A and 5B, a fin arrangement 500 is shown according to some embodiments. The side fins 530, 540 are attached to the underside of a surfcraft 510 independently of the centre fin 520. The cant angle of the side fins 530, 540 may be approximately 50°, for example. The side fin bases may be located near the centre fin base such that the leading edge of each side fin base is less than about 30% (optionally about 20% to about 5%, optionally about 13%) of the centre fin base to tip distance from the centre fin base axis 525. This separation is illustrated in FIG. 5B as X, defined as the separation of the leading edge of each side fine from the centre fin base axis 525. Thus, X may be selected so that the location of the leading edge of each of the side fin bases is less than 30% of the centre fin base to tip distance from the centre fin base axis 525.

Further embodiments will now be described with reference to FIGS. 9 to 15. A fin arrangement 1000 is shown according to some embodiments. The shape and arrangement of the fins is similar to fin arrangement 100; however, the fin arrangement 1000 comprises fin extensions 1060, 1070, 1080 projecting aftwardly from the base portion of the centre fin 1020 and each of the side fins 1030, 1040. Each of the fin extensions 1060, 1070 and 1080 may also be described as a spur, beak, horn, nose, protrusion, projection, projecting lobe or protuberance. The fin arrangement 1000 is substantially similar to fin arrangement 1600 which will be described in detail below. The inner surfaces 1038, 1048 of the side fins 1030, 1040 of the fin arrangement 1000 are entirely convex, whereas the inner surfaces 1638, 1648 of the side fins 1630, 1640 of the fin arrangement 1600 are partially concave.

Computational Fluid Dynamics (CFD) modelling was performed to compare the hydrodynamic characteristics of fin arrangement 1000 with those of a typical thruster arrangement known in the art.

FIG. 9 shows a hydrodynamic model of a typical prior art thruster setup. A main vertical central fin 2, and side fins 3 are arrayed on the underside 1 of a surfcraft. The main central fin 2 and side fins 3 are modelled from typical fins available from a commercial supplier, FCS (Fin Control Systems). A portion of the underside of the surfboard is modelled, this being the portion over which the rider is typically positioned, being 400 mm wide (in the lateral direction of the surfcraft), and 500 mm long (in the longitudinal direction of the surfcraft). Side fins were modelled at a typical cant angle of 5°. Side fins were modelled with a typical toe-in angle of 6.5°. In the lateral direction, the distance from each of the side fins to the central fin was 125 mm, which is typical for a surfcraft. In the longitudinal direction, the distance from the front edge of each side fin to the front edge of the central fin was 200 mm, which is typical for a surfcraft. The water velocity over the fins was modelled as 5 m/s, which corresponds approximately to a 1.0 metre wave height, which is a typical surfing condition. [Correlation between water velocity and wave height was sourced from a paper by Lavery et al “CFD Modelling of the Effect of Fillets on Fin Drag”]. Modelling was conducted using Multiphysics Computational Fluid Dynamics Software (ALGOR).

The results of modelling the Thruster setup are shown in FIG. 9, which is a plot of the vertical lift force distribution acting on the fin system and a portion 1 of the underside of the surfcraft. It was found that the vertical lift forces generated by the fin system were fairly small, at 16.5 N. These fin vertical lift forces are typically maximum in an area 5 (on the inner surfaces approximately one third of the base to tip distance from the underside 1 of the surfcraft and approximately one third of the chord length aft of the leading edge) on each side fin 3 shown in FIG. 9, and these lift forces arise from the cant angle of the side fins. The underside 1 of the surfcraft also generates hydrodynamic vertical lift (as a result of the planing motion of the surfcraft), and this is typically maximum in an area 4 (approximately one quarter of the side fin base chord length fore of the side fin base leading edge, and approximately half a side fin base chord length out from the side fin base axis towards the rails of the surfcraft) shown in FIG. 9. The total vertical lift of the portion of the surfcraft 1 modelled in FIG. 9 was 146 N, and therefore the total vertical lift for the portion of the surfcraft 1 and the fin system was 163 N.

Although not shown in FIG. 9, it was also found that the total horizontal drag force acting on the central fin 2 and side fins 3 was 95.8 N, and that the total horizontal drag force acting on the underside of the surfcraft 1 was 21.8 N. Therefore the total horizontal drag for the portion of the surfcraft 1 and the fin system was 118 N.

FIG. 10 is a perspective view illustrating a fin arrangement 1000 according to some embodiments. The fin arrangement 1000 comprises two symmetrical side fins 1030, 1040, being mirror images in profile and configuration. Side fins 1030, 1040 join to the central fin 1020 at an upper intersection point 1050 which may be near to the underside of the surfcraft. The centre fin 1020 has leading edge 1022 and a trailing edge 1024 converging at a tip 1026. The central fin 1020 is symmetrical about its chords and about a central plane 1025, as seen in FIG. 10, with generally hydrodynamically efficient side surfaces 1027 and 1028. The inside surface 1038, 1048 of each of the side fins 1030, 1040 is not flat and is not symmetrical with the outside surfaces 1037, 1047. Each of the side fins 1030, 1040 is fixedly coupled to the central fin 1020 (e.g. integrally formed by moulding, for example) near the base of the central fin 1020. The fin arrangement 1000 may have a coupling portion (not shown but similar to 110 in FIG. 1) extending from the base of the centre fin 1020 to allow the fin arrangement 1000 to be coupled to a surfcraft by known techniques (for example, a fin box coupling method, fin plug coupling method, or glassing-in coupling method—each as described in Australian Patent 665804, “Surf Fin Fixing System”). Each fin 1020, 1030, 1040 is swept aftwardly from base to tip.

The cant angle of the side fins 1030, 1040 may be between about 35 to about 65 degrees, optionally between about 40 and about 60 degrees, optionally about 45 to about 55 degrees, optionally about 52°, which allows for increased vertical lift compared with the “thruster” configuration in addition to the lateral lift provided by the “rail” fins of the Thruster configuration. In other words, the “Thruster” arrangement allows primarily for unilateral lift in a substantially horizontal angle to the plane of the surfcraft whereas the described multi-fin arrangement 1000 of the present application allows for lift in both the horizontal and vertical directions. It is thought that this provides for increased manoeuvrability and performance, and according to testing in variable and diverse small and large wave conditions, a feeling of increased “freedom” for the surfer. It seems that this is achieved as a direct result of the unique fin assembly geometry in the multi-fin arrangement 1000.

Computer-based hydrodynamic simulations were undertaken to measure the vertical lift, and horizontal drag forces operating on surfboards with the fin arrangement 1000 described in FIG. 10 and compared the results with the typical “thruster” arrangement in FIG. 9. The simulations indicated that in relation to the conventional “thruster” fin arrangement, the fin assembly 1000 is able to generate substantially increased vertical lift, with lower hydrodynamic drag, which is thought to lead to improved surfing performance over the “thruster” fin arrangement.

The simulations that were undertaken indicate that the increased lift of the new fin arrangement 1000 is largely generated by the fin assembly pushing water flow (and therefore pressure) upwardly onto the underside 1 of the surfcraft, and that the hydrodynamic pressure forces on the inner surfaces 1038, 1048 of the side fins 1030, 1040 are less significant. It is thought that the upwards pressure on the underside 1 of the surfboard generates far greater vertical lift forces than the vertical lift force acting directly on the fin assembly 1000 itself. Advantageously, the centroid location of the vertical upwards lift on the underside of the surfboard is located close to (and perhaps directly under) the centre of gravity of a surfer or user who may be riding the surfboard, thereby providing good balance and manoeuvrability, and reduced overturning moments.

Referring now to FIGS. 12A to 12D, the profile of a foil section of the side fin 1040 is shown. The section has been taken near to and parallel to the base axis of the side fin 1040. As shown in FIG. 12A, the inner surface 1048 is substantially flat, and slightly convex, and the outer surface 1047 is more curved and also convex. The chord 1212 is shown connecting the leading edge 1042 to the trailing edge 1044. The same profile 1210 is shown in FIG. 12B with the cambered centre line 1215 shown instead of the chord 1212. Profile 1220, shown in FIG. 12C, is an example of a more highly cambered foil for comparison only. FIG. 12D shows both foil profiles 1210 and 1220 in direct comparison. The higher camber of 1220 will perform differently during use, as discussed below.

Shown in FIG. 13 is a fin arrangement 1000, with a portion 1 of the underside of the surfcraft 400 mm wide×500 mm long, the same as the Thruster setup shown in FIG. 9. The side fins 1030, 1040 in this arrangement have a foil cross sectional profile with an inside flat face as shown by 1210 in FIG. 12A. In FIG. 13, the water pressure distribution on the fin arrangement 1000 and on the underside 1 of the surfcraft is shown. It was found that a high pressure region (generally in a zone 6) on the underside 1 of the surfcraft builds up immediately ahead of the fin arrangement 1000, and that this high pressure region creates substantial additional hydrodynamic vertical lift force. The high pressure region can also be visualised in zone 7 at the leading edge of each fin, the highest pressure being at a location approximately one third of the base to tip distance from the base axis of each fin. The zone 8 of additional lift forces created can be seen in FIG. 14 immediately fore of the centre fin base leading edge. By analysing the streamlines of the fluid flow, it was found that this zone 8 of additional vertical lift force is created by the upper faces 1037, 1047 of the side fins 1030, 1040 proximate to the underside 1 of the surfcraft pushing the water flow upwardly against the underside 1 of the surfcraft, and thereby generating the substantial additional vertical lift. The increased surface area of the side fins 1030, 1040 created by protrusions 1070, 1080 is also thought to be an important contributory factor to the increase in pressure and therefore lift on the underside 1 of the surfcraft, since these protrusions 1070, 1080 push more water against the underside 1 of the surfcraft than would be the case for a conventional tuna fin shaped side fin.

It was further found that the vertical lift forces acting directly on the fin arrangement 1000 alone were surprisingly fairly small, at 10.9 N. This finding was opposite to the teachings of the Cremin Patent Application (AU 2005220278), and thereby indicates that the fin arrangement 1000 itself does not directly generate substantial vertical lift on the underside of the surfcraft. However, the fin arrangement 1000 does cause a major increase in vertical lift force on the underside of the surfcraft, where the bases of the side fins 1030, 1040 are proximate to the base of the central fin 1020 and the underside of the surfcraft. These fin vertical lift forces are typically maximum in an area 6, immediately surrounding and ahead of the centre fin base leading edge of the fin assembly 1000 as shown in FIG. 14 in a zone 8. The vertical lift force on the underside of the surfcraft was found to be 593 N, and hence the total lift of the fin assembly 1000 and the underside of the surfcraft was found to be 604 N. Thus the total vertical lift force of the fin system 1000 in combination with the surfcraft was found to be approximately 3.7 times greater than for a typical thruster in computer simulations setup on the same 1.0 m wave. The substantial additional vertical lift forces generated by the fin assembly 1000 are therefore likely to promote improved surfing performance, including an ability for the surfer to “take off” on a wave much more quickly.

It was also found that the horizontal drag forces generated by the fin arrangement 1000 were not significantly higher than a typical thruster setup. In FIG. 15, a plot of the horizontal drag on the fin assembly 1000 is shown. Most of the hydrodynamic drag occurs in a zone 9 on the leading edges 1022, 1032, 1042 of the central fin 1020 and side fins 1030, 1040. The total drag of the fin assembly 1000 plus the drag on the portion 1 of the underside of the underside of the surfcraft was found to be 138 N, which is only 17% higher than the typical thruster setup modelled in FIG. 9. It was also found that the total drag of the fin arrangement 1000 and the underside of the surfcraft could be reduced to less than a thruster setup by making the inner surfaces 1038, 1048 of the side fins 1030, 1040 partially concave, such as the foil profile 1220 shown in FIG. 12C. Using such a profile, it was found that the total drag could be reduced to 86.9 N, which is 26% less than for a typical thruster setup. The reduced horizontal drag force from the fin assembly 1000 is likely to lead to faster surfcraft speed through the water, thereby further promoting improved surfing performance.

It was also found that locating the intersection of the side fins with the main central fins at a position further from the underside of the surfcraft had a deleterious effect on vertical lift. For example, an analysis was performed in which the location of the side fins was lowered (away from the base of the centre fin) in the vertical direction by 35 mm. This was found to reduce the vertical lift force by 18%. Hence the teachings of the Cremin Patent Application (AU 2005220278) that the side fins should join the main central fin at ⅔rds the depth of the main central fin show that positioning side fins at a location away from the base of the central fin works against the creation of substantial vertical lift. This is because the Cremin design does not allow the upper faces of the side fins 1037, 1047 to generate substantial additional hydrodynamic pressure on the underside of the surfcraft, because in the Cremin arrangement the side fins are too remote from the underside of the surfcraft to do so.

A fin arrangement 1600 according to further embodiments will now be described with reference to FIGS. 16 to 25. The fin arrangement 1600 comprises a centre fin 1620, a left side fin 1630, a right side fin 1640 (left and right from an aftward view of the fin arrangement 1600 with the centre fin extending down from the base) and a coupling portion 1690 to couple the fin arrangement 1600 to a surfcraft. The centre fin 1620 comprises a leading edge 1622, a trailing edge 1624, a base leading edge 1621, a base trailing edge 1623, a fin tip 1626, a trailing tip 1629, an extension 1660, an inner surface 1628, a left side surface 1627, and a right side surface, 1627. Each of the left side fin 1630 and right side fin 1640 comprise a leading edge 1632, 1642, a trailing edge 1634, 1644, a base leading edge 1631, 1641, a base trailing edge 1633, 1643, a fin tip 1636, 1646, a trailing tip 1639, 1649, an extension 1670, 1680, an inner surface 1638, 1648, and an outer surface 1637, 1647. The left and right side fins 1630, 1640 are enantiomorphs of each other, i.e., opposite forms or mirror images of each other. The fin arrangement 1600 is symmetrical about a central plane 1625, as shown in FIGS. 17A and 17B.

The side fins 1630, 1640 are joined to side surfaces 1627 and 1628 of the centre fin 1620 near the base of the centre fin 1620, such that each base leading edge 1631, 1641 is about 11 mm away from the base axis of the centre fin 1620 or about 7%. The cant angle is the angle between the base to tip line 1635, 1645 of each side fin 1630, 1640 and the central plane 1625 of the centre fin 1620 as illustrated in FIG. 17A. Here the cant angle is about 52°. The base to tip distance of the centre fin 1620 may be about 100 mm to 220 mm, optionally about 120 mm to 200 mm, optionally about 140 mm to 180 mm, or optionally about 160 mm. The base to tip distance of the side fins 1630, 1640 may be about 100 mm to 200 mm, optionally about 120 mm to 180 mm, optionally about 140 mm to 160 mm, optionally about 147 mm.

FIGS. 24 and 25 show the fin outline of the side fins 1630, 1640 and centre fin 1620 respectively. The leading edges 1622, 1632, 1642 have a convex curvature. The fin tips 1626, 1636, 1646 and trailing tips 1629, 1639, 1649 are rounded, and the trailing edges 1624, 1634, 1644 are substantially convex near the tip portion of each fin 1620, 1630, 1640. The trailing edges 1624, 1634, 1644 are substantially concave near the intermediate portion of each fin 1620, 1630, 1640. There is a smooth inflection point where the trailing edge 1624, 1634, 1644 transitions from convex curvature near the tip portion to concave curvature near the intermediate portion.

The extensions 1660, 1670, 1680 project aftwardly from the base portion of each fin 1620, 1630, 1640, and the trailing edges 1624, 1634, 1644 are substantially convex near the base portion of each fin 1620, 1630, 1640. Each of the fin extensions 1660, 1670 and 1680 may also be described as a spur, beak, horn, nose, protrusion, projection, projecting lobe or protuberance. Each extension 1660, 1670, 1680 defines an arc on the trailing edge 1624, 1634, 1644 such that if the line of the arc is extended, that line crosses the intermediate portion of the fin and intersects the leading edge 1622, 1632, 1642. Each extension 1660, 1670, 1680 may extend the base chord by about 10% to 50%, or optionally about 20% to about 40%, compared to a similarly shaped fin without an extension (as indicated by the dashed line 1601 in FIG. 24, for example).

There is an inflection point 1662, 1672, 1682 in the trailing edge 1624, 1634, 1644 where the extension 1660, 1670, 1680 meets the rest of the fin 1620, 1630, 1640. This inflection point may be proximate to the transition between the base portion and the intermediate portion, and may be a relatively sharp transition or interruption in curvature. In some embodiments, the inflection point may have a small radius of curvature in comparison to the radius of curvature of other parts of the leading or trailing edge. The convex trailing edges of the extensions 1660, 1670 and 1680 change at the inflection point 1662, 1672, 1682 to a concave trailing edge section of the intermediate portions. On the side surfaces 1627, 1628 and outer surfaces 1638, 1648 there is a transition step 1666, 1676, 1686 extending forward from the inflection point 1662, 1672, 1682 and continuing the curve of the trailing edge 1624, 1634, 1644 of the extension 1660, 1670, 1680. The rake angle of the side fins 1630, 1640 is about 76° and the rake angle of the centre fin 1620 is about 79°.

The plan form surface area of the side fins 1630, 1640 may be approximately 29950 mm² and the plan form surface area of the centre fin 1620 may be approximately 35380 mm², for example. The side fin extensions 1670, 1680 have a plan form area of approximately 1090 mm² and the centre fin extension 1660 has a plan form area of approximately 1140 mm². Thus, the plan form surface area of the centre fin 1620 may be greater than the plan form surface area of the side fins 1630, 1640, for example by about 15-20% of the plan form surface area of the side fins. Additionally, the plan form surface area of the centre fin extension 1660 may be greater than the plan form surface area of the side fin extensions 1670, 1680, for example by about 3-6% of the plan form surface area of the side fin extensions.

Provided below, in Table 1, are two dimensional coordinates illustrating the outline of the centre fin 1620 and side fins 1630, 1640. The coordinates are in millimetres and defined by the distance aft of the base leading edge parallel to the base axis, and the distance away from the base axis. The reference numbers for each coordinate are indicated on FIGS. 24 and 25. Reference number “0” is the base leading edge 1621, 1631, 1641, reference number “7” is the fin tip 1626, 1636, 1646, reference number “10” is the trailing tip 1629, 1639, 1649, reference number “17” is the inflection point 1662, 1672, 1682, and reference number “21” is the base trailing edge 1623, 1633, 1643.

TABLE 1 Two dimensional coordinates for fin outlines. 1620 centre 2620 centre Side fin FIG. 24 fin FIG. 25 fin FIG. 27A distance distance distance distance distance distance aft of away aft of away aft of away base from base from base from Ref leading base leading base leading base no. edge axis edge axis edge axis 0 0 0 0 0 0 0 1 16.4 35.8 18.4 37.2 21.6 42.8 2 34.2 67.2 38.5 69.0 41.1 72.6 3 55.4 96.3 61.6 98.2 65.1 102.5 4 81.7 121.2 89.1 125.2 90.7 129.2 5 105.7 135.7 112.4 141.9 119.0 152.1 6 130.7 144.6 139.7 154.6 148.0 168.2 7 148.5 146.7 166.7 159.9 173.0 176.0 8 165.1 144.2 182.9 158.8 196.4 177.4 9 176.9 137.7 196.7 153.1 215.0 171.5 10 181.5 129.1 204.3 142.7 225.0 158.3 11 176.4 117.8 200.7 134.1 221.2 146.9 12 165.7 109.8 186.8 124.1 206.7 136.3 13 150.6 99.8 171.2 113.0 189.1 125.9 14 135.2 86.2 155.9 98.1 171.4 113.2 15 123.7 69.5 143.6 80.4 154.1 95.9 16 119.4 59.8 136.2 61.8 140.6 73.7 17 116.8 50.3 133.1 45.0 133.2 45.0 18 128.0 40.8 148.0 34.6 147.7 34.9 19 140.3 25.9 160.4 24.5 158.8 26.0 20 146.7 12.5 170.6 13.2 170.2 13.9 21 149.2 0.0 175.6 0.0 175.7 0.0

FIGS. 19A to 19H show foil sections of the right side fin 1640 taken parallel to the base axis as shown in FIG. 18. The foil sections illustrate the curved surfaces of the side fin 1640. The radius of curvature of the leading edge 1642 is generally larger than the radius of curvature of the trailing edge 1644. The point of maximum thickness in each foil section is generally about one quarter of the chord length aft of the leading edge 1642, and gradually progresses to a point nearer the half-chord distance in the foil sections nearer the tip 1646. The outer surface 1647 is substantially convex, save for the transition step 1686 illustrated in FIG. 19D. The transition step 1686 is smoothed with rounded edges, and the depth of the transition step 1686 gradually decreases to zero in between sections 19D and 19E. The inner surface 1648 is convex in the base portion as seen in FIGS. 19A to 19B, but partially concave in the intermediate portion as seen in FIGS. 19D to 19G. FIG. 19H shows that the inner surface 1648 becomes convex again near the tip.

In use, as the side fins 1630, 1640 move through the water, the flow will be diverted either side of the leading edge 1632, 1642 and over the inner and outer surfaces 1638, 1648, 1637, 1647. The side fins 1630, 1640 will then act as hydrofoils and experience hydrodynamic forces in a direction normal to the outer surfaces 1637, 1647 as is known to occur in hydrofoils and aerofoils in a fluid flow.

The inner and outer surfaces 1638, 1648, 1637, 1647 are further described by the foil sections shown in FIGS. 21A to 21G. Here the sections are not taken parallel to the base axis, but at angles relative to the base axis as shown in FIG. 20 and in Table 2 in degrees. The coordinates (in millimetres) indicate the location of the leading edge of each section relative to the base axis.

TABLE 2 Section coordinates and angles FIG. number 21A 21B 21C 21D 21E 21F 21G section number 1 2 3 4 5 6 7 distance aft of base 1.2 16.4 34.2 55.5 81.7 113.1 148.4 leading edge distance away from 0.8 35.7 67.2 96.3 121.1 138.9 146.7 base axis angle relative to base 0.0 12.4 22.5 27.4 37.5 51.5 64.1 axis The outer surface 1647 is shown to be substantially convex in each of sections 21A to 21G. The inner surface 1648 is convex and substantially flat in sections 21A to 21C, partially concave in sections 21D to 21F, and convex in section 21G.

FIGS. 23A to 23G show foil sections of the centre fin 1620 taken parallel to the base axis as shown in FIG. 22. The foil sections illustrate the curved side surfaces 1627, 1628 of the centre fin 1620. The centre fin 1620 is symmetric about its chords and central plane 1625. The radius of curvature of the leading edge 1622 is generally larger than the radius of curvature of the trailing edge 1624. The point of maximum thickness in each foil section varies from base to tip between about one quarter and about one half of the chord length aft of the leading edge 1622. The surfaces 1627, 1628 are substantially convex, save for the transition step 1666 illustrated in FIG. 23C. The transition step 1666 is smoothed with rounded edges, and the depth of the transition step 1666 gradually decreases to zero in between sections 23C and 23D.

In use, as the centre fin 1620 moves through the water, the flow will be diverted either side of the leading edge 1622 and over the left and right side surfaces 1627, 1628. The symmetric profile of the centre fin 1620 will not generate a net lateral hydrodynamic force when aligned with the direction of flow: however, when the centre fin 1620 is presented to the flow at an angle of attack, a lateral lift force will be generated substantially normal to the downstream side surface.

Further embodiments will now be described with reference to FIGS. 26 to 29. The fin arrangement 2600 comprises a centre fin 2620, a left side fin 2630, a right side fin 2640 (left and right from an aftward view of the fin arrangement 2600 with the centre fin extending down from the base) and a coupling portion 2690 to couple the fin arrangement 2600 to a surfcraft. The fin arrangement 2600 is similar to the fin arrangement 1600, except that the centre fin 2620 is longer than the centre fin 1620. Such a fin arrangement 2600 may be suitable for long boards or “Malibu” surfboards, where a longer centre fin may provide improved traction or “hold”. The centre fin 2620 comprises a leading edge 2622, a trailing edge 2624, a base leading edge 2621, a base trailing edge 2623, a fin tip 2626, a trailing tip 2629, an extension 2660, an inner surface 2628, a left side surface 2627, and a right side surface, 2627. The left side fin 2630 is substantially similar to the left side fin 1630 and the right side fin 2640 is substantially similar to the right side fin 1640. The fin arrangement 2600 is symmetrical about a central plane 2625, as shown in FIGS. 27B and 27C.

The side fins 2630, 2640 are joined to side surfaces 2627 and 2628 of the centre fin 2620 near the base of the centre fin 2620, such that each base leading edge 2631, 2641 is about 11 mm away from the base axis of the centre fin 2620 or about 6%. The cant angle is the angle between the base to tip line 2635, 2645 of each side fin 2630, 2640 and the central plane 2625 of the centre fin 2620 as illustrated in FIG. 27A. Here the cant angle is about 52°. The base to tip distance of the centre fin 2620 may be about 120 mm to 240 mm, optionally about 140 mm to 220 mm, optionally about 160 mm to 200 mm, or optionally about 177 mm. The base to tip distance of the side fins 2630, 2640 may be about 100 mm to 200 mm, optionally about 120 mm to 180 mm, optionally about 140 mm to 160 mm, optionally about 147 mm.

FIG. 27 shows the fin outline of the centre fin 2620. The leading edge 2622 has a convex curvature. The fin tip 2626 and trailing tip 2629 is rounded, and the trailing edge 2624 is substantially convex near the tip portion. The trailing edge 2624 is substantially concave near the intermediate portion. The extension 2660 projects aftwardly from the base portion, and the trailing edge 2624 is substantially convex near the base portion. There is an inflection point 2662 in the trailing edge 2624 where the extension 2660 meets the rest of the fin 2620. On the side surfaces 2627, 2628 there is a transition step 2666 extending forward from the inflection point 2662 and continuing the curve of the trailing edge 2624 of the extension 2660. The rake angle of the centre fin 2620 is about 72°.

The plan form surface area of the side fins 2630, 2640 may be approximately 29950 mm² and the plan form surface area of the centre fin 2620 may be approximately 39480 mm². The side fin extensions 2670, 2680 have a plan form area of approximately 1090 mm² and the centre fin extension 2660 has a plan form area of approximately 1140 mm². Thus, the plan form surface area of the centre fin 2620 may be greater than the plan form surface area of the side fins 2630, 2640, for example by about 15-20% of the plan form surface area of the side fins. Additionally, the plan form surface area of the centre fin extension 2660 may be greater than the plan form surface area of the side fin extensions 2670, 2680, for example by about 3-6% of the plan form surface area of the side fin extensions.

Provided above, in Table 1, are two dimensional coordinates illustrating the outline of the centre fin 2620. The coordinates are in millimetres and defined by the distance aft of the base leading edge parallel to the base axis, and the distance away from the base axis. The reference numbers for each coordinate are indicated on FIG. 27A. Reference number “0” is the base leading edge 2621, reference number “8” is nearest the fin tip 2626, reference number “10” is the trailing tip 2629, reference number “17” is the inflection point 2662, and reference number “21” is the base trailing edge 2623.

FIGS. 29A to 29H show foil sections of the centre fin 2620 taken parallel to the base axis as shown in FIG. 28. The foil sections illustrate the curved side surfaces 2627, 2628 of the centre fin 2620. The centre fin 2620 is symmetric about its chords and central plane 2625. The radius of curvature of the leading edge 2622 is generally larger than the radius of curvature of the trailing edge 2624. The point of maximum thickness in each foil section varies from base to tip between about one quarter and about one half of the chord length aft of the leading edge 2622. The surfaces 2627, 2628 are substantially convex, save for the transition step 2666 illustrated in FIG. 29C. The transition step 2666 is smoothed with rounded edges, and the depth of the transition step 2666 gradually decreases to zero near section 29D.

Provided below, in Table 3, are the thickness to chord ratios (maximum thickness perpendicular to the chord: leading edge to trailing edge distance) for each of the evenly spaced parallel foil sections shown for the side fins 1630/2630, 1640/2640 and centre fins 1620 and 2620.

TABLE 3 Thickness to chord ratios side fin 1620 short centre fin 2620 long centre fin section max. chord T/C max. chord T/C max. chord T/C no. thickness length (%) thickness length (%) thickness length (%) 1 10.9 147.8 7.4 10.5 174.1 6.0 10.5 174.1 6.0 2 10.4 129.6 8.0 9.9 144.8 6.8 9.9 140.4 7.1 3 9.3 97.4 9.5 9.2 107.3 8.6 9.1 105.1 8.7 4 7.8 88.0 8.9 8.5 98.1 8.7 8.3 96.4 8.6 5 6.7 89.2 7.5 7.8 94.4 8.3 7.7 95.6 8.1 6 5.8 98.9 5.9 7.1 98.5 7.2 7.1 107.6 6.6 7 4.3 70.6 6.1 5.3 81.2 6.5 5.4 88.7 6.1 mean 7.6 7.4 7.3

The dimensions and proportions of the described embodiments are given for the purpose of illustration only. The dimensions and proportions of the fin arrangement may be varied for different performance requirements or differently sized surfcraft.

In embodiments of fin arrangement 1000, 1600 and 2600, there may be multiple inflection points of the curvature of the trailing edge of each fin that has an extension. One of those inflection points may be formed as a relatively sharp corner or transition area.

Further embodiments may include any combination of the features described in the various embodiments described above. For example, embodiments may include fin arrangements in which the centre fin 1620 or 2620 of fin arrangement 1600 or 2600 is combined with the side fins 130, 140 of fin arrangement 100, or the centre fin 120 of fin arrangement 100 is combined with the side fins 1630, 1640 of fin arrangement 1600.

Embodiments described herein may be applied to different types of surf craft, such as different types of surfboards, boards for windsurfing or other wave-riding or water-surface-riding craft. Surfcraft of different sizes may use versions of the fin arrangements described herein that are scaled to suit the size of the surfcraft. Embodiments may also employ fins of somewhat varying proportions to those described herein without departing from the spirit and scope of this disclosure.

It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the above-described embodiments, without departing from the broad general scope of the present disclosure. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive. 

What is claimed is:
 1. A fin arrangement for a surf craft, comprising: a base coupling portion to couple the fin arrangement to the surf craft; a central fin having a central fin base portion, the central fin coupled to the base coupling portion to extend substantially perpendicularly away from an underside of the surf craft; a first side fin coupled to the central fin base portion and extending away from the central fin and the surf craft at a first acute angle to the central fin; and a second side fin coupled to the central fin base portion and extending away from the central fin and the surf craft at a second acute angle to the central fin, the second acute angle being substantially the same as the first acute angle.
 2. A fin arrangement for a surf craft, comprising: a central fin to extend substantially perpendicularly away from an underside of the surf craft; a first side fin coupled to one of: a central fin base portion of the central fin, and an underside of the surf craft at a position closely adjacent to the central fin base portion, the first side fin extending away from the central fin and the surf craft at a first acute angle to the central fin; and a second side fin coupled to one of: the central fin base portion, and to the underside of the surf craft at a position closely adjacent to the central fin base portion, the second side fin extending away from the central fin and the surf craft at a second acute angle to the central fin, the second acute angle being substantially the same as the first acute angle.
 3. The fin arrangement of claim 2, wherein the first and second acute angles are between about 35° and about 65°.
 4. The fin arrangement of claim 3, wherein the first and second acute angles are between about 40° and about 60°.
 5. The fin arrangement of claim 4, wherein the first and second acute angles are between about 45° and about 55°.
 6. The fin arrangement of claim 5, wherein the first and second acute angles are between about 48° and about 52°.
 7. The fin arrangement of claim 2, wherein the central fin has a lateral width of about 9 mm to about 15 mm at a base of the central fin.
 8. The fin arrangement of claim 2, wherein a lateral width of the first side fin and the second side fin is non-uniform in a fore-aft direction and in a base-to-tip direction.
 9. A fin arrangement for a surf craft, comprising: a base portion to couple the fin arrangement to the surf craft; a central fin coupled to the base portion to extend substantially perpendicularly away from an underside of the surf craft; a first side fin coupled to the central fin and extending away from the central fin and the surf craft at a first acute angle to the central fin; and a second side fin coupled to the central fin and extending away from the central fin and the surf craft at a second acute angle to the central fin, the second acute angle being substantially the same as the first acute angle, wherein at least the central fin comprises a projecting lobe extending rearwardly from a base of the central fin.
 10. The fin arrangement of claim 9, wherein the projecting lobe defines at least part of a trailing edge of the central fin.
 11. The fin arrangement of claim 10, wherein the trailing edge along the projecting lobe has a convex curvature.
 12. The fin arrangement of claim 10, wherein the trailing edge has a sharp inflection point at a location adjacent the projecting lobe.
 13. The fin arrangement of claim 10, wherein the trailing edge of the central fin has multiple inflection points along its length.
 14. The fin arrangement of claim 9, wherein the first side fin and second side fin each comprise a projecting lobe extending rearwardly from respective bases of the first and second side fins.
 15. The fin arrangement of claim 14, wherein the projecting lobe of each of the side fins has a plan form surface area less than a plan form surface area of the central fin.
 16. A fin arrangement for a surfcraft, comprising: a central fin, a first side fin and a second side fin, wherein a cant angle of each side fin is between 30° and 65°, and wherein a leading edge of a base of each side fin is positioned at a distance from a base axis of the central fin, wherein the distance is less than 30% of a base to tip distance of the central fin.
 17. The fin arrangement of claim 2, wherein each of the first and second side fins comprises a projecting lobe extending rearwardly from respective bases of the first and second side fins.
 18. The fin arrangement of claim 2, wherein inner faces of the first and second side fins are at least partially concave.
 19. The fin arrangement of claim 2, wherein a leading edge of each of the central and first and second side fins has a convex curvature along its length.
 20. The fin arrangement of claim 2, wherein a trailing edge of each of the central and first and second side fins has a concave curvature along at least part of its length.
 21. A surfcraft comprising the fin arrangement of claim
 2. 